Reactivity of methane in a nitrogen discharge afterglow

This work is devoted to the study of the reactivity of CH 4 in a nitrogen afterglow. We show that CH 4 reacts efficiently in the nitrogen afterglow producing the simple radicals CH 3 , CH 2 or recombined species containing C-N bonds in the form of C 2 N 2 , CN or HCN. A reaction rate constant ranging between 1 × 10 -18 and 6 × 10 -18 m 3 s -1 is found for the global reaction producing mainly CH 2 and CH 3 radicals. Investigations are also performed in order to determine which `active' nitrogen species are involved in the dissociation mechanism of CH 4 in the nitrogen afterglow. To do this we study elementary reactive processes between CH 4 and specific nitrogen species such as N 2 (A 3 Σ u + , v ) and N atoms. N 2 (A 3 Σ u + , v ) is selectively produced by energy transfer from Ar( 3 P 2 ) metastable species to N 2 , before reacting with CH 4 . We show that this reaction is efficient and leads mainly to the formation of CH 3 and C 2 H 6 . According to the literature, the higher the vibrational level is the faster is the rate of the vibrational relaxation of N 2 (A 3 Σ u + , v ). We also study the reaction of CH 4 with N atoms in the nitrogen afterglow. We measure a reaction rate constant ranging between 2 × 10 -18 and 6 × 10 -18 m 3 s -1 . This value is larger than the value reported elsewhere in the literature and close to the value given for the reaction of CH x <4 with N atoms. So we propose the following reaction mechanism for the dissociation of CH 4 in a nitrogen afterglow. First CH 4 reacts with vibrationally excited metastable species N 2 (A 3 Σ u + , v ) coming from the nitrogen discharge that produces CH x <4 simple radicals. Then these radicals react with N atoms producing hydrogen cyanide, cyanogen and hydrocarbons.